1. Field of the Invention
This invention relates to the coating of fibers.
2. Description of the Prior Art
The production of optical fiber usually involves applying one or more coatings to a bare optical fiber. Most optical fibers comprising silica are drawn at high temperature from a preform. One method of preform production is the modified chemical vapor deposition (MCVD) process, as described in U.S. Pat. No. 4,217,027, coassigned with the present invention. Other methods of preform production are also utilized. As the fiber is drawn from the preform, it is typically coated with at least one layer of material. In a dual layer-coated fiber, a first layer typically comprises a relatively soft material, including for example, a silicone or a hot melt wax material for reduced microbending losses. A second layer deposited thereon typically comprises an ultraviolet-cured or thermally cured polymeric material for high strength and abrasion resistance. Other materials and coating combinations are possible. Current draw speeds typically run approximately 1 meter/second, but for economical optical fiber production, it is desired to obtain even higher drawing speeds.
On-line coating of optical fibers is usually done with open cut applicators. Typical devices consist of glass or metal containers, sometimes of a split configuration to avoid a threading problem, wherein the fiber enters the liquid reservoir through the free surface at the top and exits through a rigid or flexible orifice at the bottom. One typical coating apparatus is described in U.S. Defensive Publication No. T963002, coassigned with the present application. However, in such open cup applicators, the wetting of the glass surface depends on conditions at the entering meniscus, the viscosity of the coating medium, and on the static head available in the reservoir. Concentricity and surface quality of the coating are largely dependent on flow in the exit nozzle. At higher drawing speeds, typically above about 2 or 3 meters/second, it has been found that bubble entrapment and incomplete wetting degrade the coating quality. In addition, the centering of the fiber in the exit nozzle is due only to rather weak forces in the case of a convergent nozzle geometry, producing rather poor centering of the fiber in the nozzle.
In other coating fields, for example plastic coating for insulation on copper wire, the copper wire is drawn through a close-fitting die into a coating chamber, wherein the polymer coating is applied at high speeds and high pressures. However, in the case of optical fibers, care must be taken to avoid contact of the bare fiber with the coating apparatus. Therefore, a very close-fitting entrance to the coating chamber is not possible, and other means must be found to obtain high quality, high speed coating of optical fibers.